Potterton Netaheat...revisited

Hello.
Last week I posted about problems with my 15 yrs old Netaheat Electronic 16/22 (flame going out then continuous ignition cycles). That problem was solved by replacing the PCB with a recon unit. Unfortunately that seems to have brought on another problem- boiling!
After being on for about 1.5 hours in its newly fixed state, there was a loud bubbling sound from the pipes and the sound of water pouring out of the header tank overflow. The CORGI guy who fitted the PCB insists that it can't be anything to do with the new (well, recon) PCB and must be the thermostat. This seems a bit too much of a coincidence?
The thermostat is connected to the PCB by two wires which (according to the schematic stuck to the boiler) connect to either side of the switch within the thermostat. I've tested the boiler with these two wires disconnected, and also with them connected to each other. On both occasions the boiler is happy to keep on going. Doesn't this mean that the PCB is ignoring the input from the thermostat- or am I missing something?
Attemped ASCII art of thermostat wiring...(the top section doesn't seem to be used for anything as the two connection block wires do not connect on to anything else?
| Mains Live | -------- | | | Conn | o | Conn block (unused?) block | o--|----- (unused?) | / | ------|-o | | | | o--__o | | | | | --------- | | | PCB PCB (these are the wires I've been playing with) L1 L2
Any advice gratefully accepted...
Thanks, Dave
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Dave Ramage wrote:

I sure that there is a running thermostat which is the one with the knob and numbers. If this does not cut out the boiler well before it boils then there is a problem.
IIRC there is also a high limit over heat cut out - which is manually resettable.
Both operate from a copper bulb inserted into a pocket into the heat exchanger.
The fact that neither is operating is cause for concern.
Whether or not the problem is coincidence the problem needs attention.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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Ed,
Thanks for your input. I did some testing with a multimeter and concluded that the thermostat *was* openning after about 20 minutes, but the PCB was ignoring this. CORGI guy is fitting a new PCB- this time a new one, as the recon PCBs don't seem to be relyable!
The boiler doesn't appear to have an overheat cut out- there is an add-on part available, but this is only fitted standard if used in a sealed system. After this though, I might fit one any way!
Cheers, Dave.
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Dave Ramage wrote:

thermostat is a basic safety check... but we all make mistakes...
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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I take it that the recon pcb you had wasn't one of mine, but anyway, the stat cuts the power to the pcb, so it's highly unlikely that the pcb is at fault. It sounds like someone's mis-wired it.
--
geoff

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writes

I have used two recon PCBs. Both have had faults, therefore I don't consider them to be relyable. That conclusion seems to be entirely sensible based on the available data?
I'm sure the quality control at CETltd is better than my supplier (can't remember what it said on the box, but wasn't CET), but I'm still happier now that I have a Potterton original installed.
The PCB was wired correctly (first thing I checked). Replacing the PCB fixed the fault, so I'm certain that that was the problem. It looked to me like the loop through the thermostat was shorted out somewhere on the PCB.
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